A construction machine having a working tool unit, such as a bucket at a front side of a cab as shown in FIG. 12, comprises booms 101 which are rockingly mounted, an arm 102 which is rockingly mounted on the booms 101; a working tool unit 100 such as, for example, a bucket 103 installed on the arm 102; boom cylinders 104; an arm cylinder 105 or a bucket cylinder 106 operated by an operator, whereby the working tool unit is actuated to carry out work. However, the attitude of such construction machine during transportation is unstable and adversely affects the efficiency of transportation because its overall length L and overall height H are large and its center of gravity is deviated. Moreover, visibility during travel is unsatisfactory. Therefore, those inventions disclosed in Japanese Patent Application Disclosure SHO. 53-30105 and SHO. 64-66319 have been proposed. An arm of a working tool unit disclosed in Japanese Patent Application Disclosure SHO. 53-30105 is accommodated for transportation by being positioned past a dead center located beyond a working position limit and being collapsed right above the vehicle body and at a side of the cab. For transporting the working tool unit disclosed in Japanese Patent Application Disclosure SHO. 64-66319, one of the two connector pins for a first boom and a second boom is removed, and the first boom and the second boom are collapsed, one above another, by rotating first boom rearwardly at the side of the cab. For working, the connector pin, which had been removed, is installed again, and the first and second booms are operated as a single integrated boom.
The working tool unit disclosed in Japanese Patent Application Disclosure SHO. 53-30105 has insufficient stability during transportation, since its overall height is high and therefore its center of gravity is also positioned high, because the working tool unit is folded right above the vehicle machine. In addition, when the working tool unit is halted to be inoperable at the upper dead point, there is a problem unless a specific control or an operation using the inertia of the working tool unit is performed, since the first boom cylinder passes through the upper dead point during switching over from a state of storage to a state of working.
The working tool unit disclosed in Japanese Patent Application Disclosure SHO. 64-66319 requires the removal and remounting of the connector pin each time the attitude of the working tool unit needs to be changed from a working attitude to a transportation attitude or from a transportation attitude to a working attitude, and therefore the work for changing the attitude is troublesome and requires a number of man-hours. If the operation is faulty when the connector pin is installed for changing the attitude of the working tool unit from a transportation attitude to a working attitude, the bucket is disengaged from any bucket engagement device, since a cylinder is not provided between the first boom and the second boom, although the bucket can be properly engaged with the bucket engagement device and the booms can be normally extended and contracted if the boom cylinders and the arm cylinder are simultaneously operated in position. In this case, the first boom and the second boom, which are connected only with one connector pin, can never be operated, and the position of the working tool unit can neither be changed to the transportation attitude nor to the working attitude unless it is supported by a crane.
In addition, lately the traffic congestion has been more intensified due to the increase in the number of vehicles, and an improvement of transportation efficiency has been strongly demanded. Particularly, it has been demanded that construction machines, which need to travel by themselves to working sites, be capable of traveling at higher speeds. In this case, it is also demanded that the construction machines have stability in high speed travel, turning performance, reduction of overall length, satisfactory visibility, and smaller turning circle.